With the battery becoming increasingly popular as a power source, there has been an equally strong demand, maximizing battery life. Battery imbalance (that means mismatch of the state of charge of battery cells in a battery pack) in large lithium-ion battery pack is a problem. This problem is caused by differences in manufacturing processes, working conditions and battery aging. The imbalance may reduce the total capacity of the battery pack, and may damage the battery pack, too. The imbalance of the battery from charging to discharging state can not be traced. If it is not closely monitored, the battery may leads to overcharge or over-discharge, which will permanently damage the battery. For car-used battery, the above issue becomes more significant, especially for lithium batteries.
In general, the voltage of each lithium battery is not high and the current is not strong enough. Therefore, lithium iron phosphate batteries are developed to enhance voltage and current. However, when such batteries are connected in series and parallel, it is also easily cause “imbalance” to shorten caused discharge time of the battery pack (i.e., the lifetime time is shortened), and accelerate the aging of the battery pack.
BMS (Battery Management System) is a protection mechanism. It is used for monitoring the whole battery pack. When the battery pack is detected to have some battery cells which are imbalance, it will turn off the entire battery pack system and the battery pack no longer supply power. The BMS uses a balance loop to achieve the above goal. Most of the balance loops are used during battery pack charging. It is scarcely used in battery pack discharge. The balance loop can be divided into two type, the active type and passive type. The active type balance loop has advantage of high efficiency, but it also has drawback of high cost and big size. Passive type balance loop has advantages of cheap cost. However, the disadvantage is low efficiency which will generate heat. Balance effect is limited. Although the active type is highly efficient, considering the price, most of the battery packs are chosen to built-in passive type balance loop.
For an electron vehicle, it is not desired that the battery pack which is used is too large. Therefore, the BMS of the battery should be designed properly. A prior art can be a good example. Please refer to FIG. 1. US Patent Application No. 20120105001 discloses a battery management system. The battery management system includes several subsystem blocks, an energy storage master unit 100, and battery pack systems 104. The energy storage master may interface with the vehicle master controller (ZR32-A) 101 with a pass through from the energy storage master 100 by way of CAN or other communication method to an external charger 102. The vehicle master controller 101 may interface with the external charger 102 either directly or through a charging station interface. The energy storage system may include several strings of batteries 103 in an electric vehicle. Within each of these strings 103, there may be packs 104, and each pack is comprised of several battery modules. The battery packs 104 may communicate to the energy storage master 100 by way of a second CAN bus. Two battery packs 104 may make up a string 103. The packs may be controlled by a pack master, which may communicate with the energy storage master 100 using a single CAN bus for the entire system. Each pack master may communicate with its local module unit using an serial peripheral interface (SPI) bus. The local module unit and pack master communications may be isolated. The battery packs containing 10 prismatic battery cells each. There are 2 packs per string, and a variable number of strings per vehicle (typically 3 to 4).
According to the invention, the battery pack comes with the battery cell balancing system would still be bulky. However, it provides people a way to reduce the size: taking the battery cell balancing system out of the battery pack. If one or more sets of battery cell balancing systems can be setup out of battery packs, the cost of battery cell balancing system can be saved. In addition, the battery pack size can be smaller. More economic and convenient, a charging station can combine the battery cell balancing system to provide power to battery pack while monitors state of each battery cell or pack. It is the point the inventor created the present invention.